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2D layered crystals for thermoelectric nanocomposites and devices

Author

Ng, Hong Kuan

Date of Issue

2016

School

School of Physical and Mathematical Sciences

Abstract

The field of thermoelectrics has displayed great potentiality as one of the viable technologies for cleaner and more sustainable energy sources due to the escalating energy crisis in the 21st century. Thermoelectric (TE) devices allow waste thermal energy (heat) to be directly converted into electrical energy from heat sources via the Seebeck effect. Conversely, the Peltier effect can induce heating or cooling effects by converting electrical energy into thermal energy. The key advantages of TE devices over conventional power sources are its reliability and negligible greenhouse gas emissions. Currently, many efforts are placed into pushing its figure of merits (ZT) to greater than 3 in order for TE applications to attain wider economical applications.
This research project focuses on the Bismuth Telluride (Bi_2 Te_3 ) material which exhibits the best TE performance in room temperatures. Nanocomposites (NC) of the n-type Bismuth Telluride Selenide (Bi_2 Te_(3-x) Se_x) alloys with different Se compositions and excess tellurium (Te) were prepared via a bottom-up solution synthesis and spark plasma sintering (SS-SPS) method. The Bi_2 Te_(3-x) Se_x alloy with the highest ZT was then synthesized with extra dopants (Cu,In,Sn). The SS-SPS method is time-, cost- and energy-efficient compared to other common preparation methods. Measurements in transport properties of the synthesized NCs portrayed enhanced power factor and reduced lattice thermal conductivity, revealing potentiality to outperform n-type commercial ingots.
The SS-SPS p-type Bismuth Antimony Telluride (Bi_0.5 Sb_1.5 Te_3) NC achieved a maximal ZT of 1.59, which is substantially better than the p-type commercial ingots with ZT of ~1.1. Unicouple cooling devices were constructed to confirm these ZT values by comparing their maximum temperature differences attainable. The unicouple constructed with the SS-SPS p-type NC demonstrated a higher TE efficiency with a maximum temperature difference of ~5℃ higher than the unicouple constructed with both n- and p-types commercial ingots.